Coupled metallic ring gap waveguide

Employing the surface plasmon polaritons (SPPs), a kind of coupled metallic squareness ring waveguide structure is presented. Its properties has been analyzed with the finite different time domain method and the coupling length has been derived from the coupled mode theory. It is demonstrated that the SPPs excited by the light with different wavelength will come out from different output port due to different coupling length. By appropriately designing the structure, it can be utilized to realize some optics devices such as multiple-wavelength sorter and beam splitter. This will break through the diffraction limit of traditional optical devices.     

Wavelength demultiplexing structure based on arrayed plasmonic slot cavities

A compact wavelength demultiplexing structure based on arrayed metal-insulator-metal (MIM) slot cavities is proposed and demonstrated numerically. The structure consists of a bus waveguide perpendicularly coupled with a series of slot cavities, each of which captures SPPs at the resonance frequency from the bus waveguide and tunes the transmission wavelength by changing its geometrical parameters. A cavity theory model is used to design the operating wavelengths of the structure. Moreover, single band transmission of each channel and the adjustable transmission bandwidth can be obtained by altering the drop waveguide positions and the coupling distance. The proposed arrayed slot cavity-based structure could be utilized to develop ultracompact optical wavelength demultiplexing device for large-scale photonic integration.     

基于耦合开口方环共振空腔的可控法诺共振研究

基于表面等离子激元理论与金属-介质-金属波导结构提出一个由开口方环共振空腔、挡板及MIM波导组成的波导结构,并使用有限元方法系统地研究了该结构的透射特性.仿真计算结果表明:该结构可以产生法诺共振现象,其共振波长可以通过改变开口方环空腔的长度及开口大小进行调节,该结构敏感度可达1 600nm/RIU,品质因数为1.31×10~5.此外,通过调整方环共振空腔上开口的位置,在波导中产生了双重法诺共振现象,其敏感度可达1 700nm/RIU,品质因数为8.3×10~4.该结构有望在光学集成回路,尤其是纳米生物传感器方面得到比较广泛的应用.     

Surface plasmon resonance and field confinement in graphene nanoribbons in a nanocavity

In this work, we demonstrate surface plasmon resonance properties and field confinement under a strong interaction between a waveguide and graphene nanoribbons (GNRs), obtained by coupling with a nanocavity. The optical transmission of a waveguide–cavity–graphene structure is investigated by finite-difference time-domain simulations and coupled-mode theory. The resonant frequency and intensity of the GNR resonant modes can be precisely controlled by tuning the Fermi energy and carrier mobility of the graphene, respectively. Moreover, the refractive index of the cavity core, the susceptibility χ⁽³⁾ and the intensity of incident light have little effect on the GNR resonant modes, but have good tunability to the cavity resonant mode. The cavity length also has good tunability to the resonant mode of cavity. A strong interaction between the GNR resonant modes and the cavity resonant mode appears at a cavity length of L₁ = 350 nm. We also demonstrate the slow-light effect of this waveguide–cavity–graphene structure and an optical bistability effect in the plasmonic cavity mode by changing the intensity of the incident light. This waveguide–cavity–graphene structure can potentially be utilised to enhance optical confinement in graphene nano-integrated circuits for optical processing applications.     

含双挡板金属-电介质-金属波导耦合方形腔的独立调谐双重Fano共振特性

基于表面等离子激元在亚波长结构的传输特性,设计了一种含双挡板金属-电介质-金属波导耦合两个方形腔的结构.由F-P谐振腔产生的宽谱模式与两个方形谐振腔产生的两个窄谱模式发生干涉作用,形成了独立调谐的双重Fano共振,而且可以通过改变两个方形腔的大小及填充介质实现双重Fano共振的独立调谐.基于耦合模理论,定性分析了该结构产生双重Fano共振的机理.利用有限元仿真的方法,定量分析了结构参数对可独立调谐双重Fano共振和折射率传感特性的影响.结果表明,优化参数后该结构的灵敏度分别高达1020和1120 nm/RIU, FOM值分别高达3.59×10~5和1.17×10~6.该结构可为超快光开关、多功能高灵敏度传感器和慢光器件的光学集成提供有效的理论参考.     

Adiabatic transfer of surface plasmons in non-Hermitian graphene waveguides

We investigate the energy transfer of surface plasmon polaritons (SPPs) based on adiabatic passage in a non-Hermitian waveguide composed of three coupled graphene sheets. The SPPs can completely transfer between two outer waveguides via the adiabatic dark mode as the waveguides are lossless and the coupling length is long enough. However, the loss of graphene can lead to breakdown of adiabatic transfer schemes. By utilizing the coupled mode theory, we propose three approaches to cancel the nonadiabatic coupling by adding certain gain or loss in respect waveguides. Moreover, the coupling length of waveguide is remarkably decreased. The study may find interesting application in optical switches on a deep-subwavelength scale.     

Plasmonically induced reflection in metal-insulator-metal waveguides with two silver baffles coupled square ring resonator

A plasmonic waveguide coupled system that is composed of a square ring cavity and a metal–insulator–metal(MIM)waveguide with two silver baffles is proposed. The transmission and reflection properties of the proposed plasmonic system are investigated numerically using the finite element method. The normalized H_z field distributions are calculated to analyze the transmission mode in the plasmonic system. The extreme destructive interference between light mode and dark mode causes plasmonically induced reflection(PIR) window in the transmission spectrum. The PIR window is fitted using the coupled mode theory. The analytical result agrees with the simulation result approximately. In addition, the PIR window can be controlled by adjusting structural parameters and filling different dielectric into the MIM waveguide and the square ring cavity. The results provide a new approach to designing plasmonic devices.     

统一非对称光波导横向耦合模理论分析

光波导横向耦合模理论包含正交耦合模理论和非正交耦合模理论两大类, 为了追求形式上的完美, 两类理论没有统一的解析解, 并且都没有对非匹配耦合系统中耦合功率的非对称性进行深入研究. 本文一方面由Helmholtz方程出发推导出了一种新型的非正交耦合模方程, 并对两类理论中的耦合模方程进行了统一处理和求解, 得到了一种统一的解析解; 另一方面根据所得到的统一的解析解对非匹配耦合系统中耦合功率的非对称性进行了详细研究, 计算结果表明非匹配耦合系统中的最大互耦合功率和最小自耦合功率均可用统一的解析解进行近似计算. 关键词： 光波导 耦合模理论 耦合器 最大耦合功率     

Ultra-compact graphene plasmonic filter integrated in a waveguide

Graphene plasmons have become promising candidates for deep-subwavelength nanoscale optical devices due to their strong field confinement and low damping. Among these nanoscale optical devices, band-pass filter for wavelength selection and noise filtering are key devices in an integrated optical circuit. However, plasmonic filters are still oversized because large resonant cavities are needed to perform frequency selection. Here, an ultra-compact filter integrated in a graphene plasmonic waveguide was designed, where a rectangular resonant cavity is inside a graphene nanoribbon waveguide. The properties of the filter were studied using the finite-difference time-domain method and demonstrated using the analytical model. The results demonstrate the band-pass filter has a high quality factor(20.36) and electrically tunable frequency response. The working frequency of the filter could also be tuned by modifying the cavity size. Our work provides a feasible structure for a graphene plasmonic nano-filter for future use in integrated optical circuits.     

Multi-wavelength filtering in side-coupled cascaded-cavities systems

We theoretically investigate an optical system, which consists of a waveguide side-coupled to a cascade of cavities, the filtering properties by the time domain coupled-mode theory. In such a system, the filter operating bandwidth and the width of transmission peaks are completely determined by the mutual coupling coefficients between the cavities. The characteristics are numerically demonstrated in photonic crystals by the finite-difference time-domain method.     

光子晶体方形谐振器波分解复用的特性

光通信系统设计中波分解复用器是分离光信号的一种关键部件,用于分离光信号的微型谐振器特性直接关系到波分复用(WDM,wavelength division multiplexing)解复用系统的工作性能。在二维光子晶体中逐步优化设计了基于光子晶体方形谐振器(PCSR,photonic crystal square resonator)的单信道WDM解复用结构,借助于耦合模理论(CMT,coupled-mode theory)定性分析了波导与谐振腔结构的电磁波耦合相互作用,并用时域有限差分法(FDTD,finite-difference time-domain)数值模拟了其结构工作特性。结果表明:基于PCSR设计的单输出端口WDM解复用结构在设计的参数范围中具有单谐振峰、中心波长宽调谐范围(1 501.4 nm~1 591.0 nm)、通带带宽窄(3.3 nm~9.1 nm)的特性。该结构可应用于WDM解复用光通信系统设计和光路集成设计等方面。     

基于耦合T型空腔的多重法诺共振现象

提出由T型空腔和挡板组成的两种金属-电介质-金属(MIM)波导结构,分别为:正T型空腔结构和倒T型空腔结构,并应用有限元法系统地研究了该结构的透射特性.对于正T型空腔结构,仿真结果出现了双重法诺共振现象,并且共振波长可以通过改变T型空腔长度和高度进行调节.该结构有助于设计成敏感度达到1 620nm/RIU、品质因数为5.4×10~4的纳米传感器.对于倒置T型空腔,在波导中产生了多重法诺共振现象,其敏感度可达1 560nm/RIU,品质因数为9.37×104.该结构有望在光学集成回路,特别是纳米传感器、光束分路器方面具有广泛应用.     

Resonant cavity based antireflection structures for surface plasmon waveguides

The concept of antireflection coating in the theory of multilayer films is introduced to the two-dimensional metal–insulator–metal (MIM) structures to realize total transmission of optical energy at the waveguide discontinuities. The antireflection structure consists of a resonant cavity which is constructed by changing the insulator width of the waveguide. A numerical method is used to achieve the optimal design directly. A T-splitter with zero reflection is proposed, utilizing a cavity structure in the input waveguide. A transformer with enhanced transmission between different waveguides is presented for further validating the efficiency and generality of these cavity based antireflection structures. The simulation results show that such a structure can realize a perfect antireflection function.     

Dynamically tunable multiband plasmon-induced transparency effect based on graphene nanoribbon waveguide coupled with rectangle cavities system

A dynamically tunable multiband plasmon-induced transparency (PIT) effect in a series of rectangle cavities coupled with a graphene nanoribbon waveguide system is investigated theoretically and numerically by tuning the Fermi level of the graphene rectangle cavity. A single-PIT effect is realized using two different methods: one is the direct destructive interference between bright and dark modes, and the other is the indirect coupling through a graphene nanoribbon waveguide. Moreover, dual-PIT effect is obtained by three rectangle cavities side-coupled with a graphene nanoribbon waveguide. Results show that the magnitude of the dual-PIT window can be controlled between 0.21 and 0.74, and the corresponding group index is controlled between 143.2 and 108.6. Furthermore, the triple-PIT effect is achieved by the combination of bright-dark mode coupling and the cavities side-coupled with waveguide mechanism. Thus, sharp PIT windows can be formed, a high transmission is maintained between 0.51 and 0.74, and the corresponding group index is controlled between 161.4 and 115.8. Compared with previously proposed graphene-based PIT effects, the size of the introduced structure is less than 0.5 μm². Particularly, the slow light effect is crucial in the current research. Therefore, a novel approach is introduced toward the realization of optical sensors, optical filters, and slow light and light storage devices with ultra-compact, multiband, and dynamic tunable.     

Trapping of surface plasmon polaritons in a multiple-teeth-shaped waveguide at visible wavelengths

We numerically and theoretically demonstrate that a metal–insulator–metal (MIM) waveguide with multiple-teeth-shaped graded depths can strongly slow light as the propagation velocities of surface plasmon polaritons (SPPs) are reduced over a large frequency bandwidth at visible wavelengths domain. Since the wavelength of the trough of transmission is dependent on the depth of the tooth-shaped dielectric in the MIM waveguide, the guided SPPs at different frequencies can be localized at different spatial positions of the multiple-teeth-shaped graded depths MIM waveguide, which can be proved by the scattering matrix method. The separation between trapped waves can be tuned by changing the grade of the tooth-shaped depths and the lifetime of SPPs in the waveguide may be long enough for some meaningful nano-photonic applications.     

基于硅基光子器件的Fano共振研究进展

硅基光子技术的发展为新型微纳光学功能器件和片上系统提供了高可靠、高精度的实现手段.采用硅基光子技术构建的具有连续(准连续)模式微腔与离散模式的微腔耦合产生的Fano共振现象得到了广泛关注.Fano共振光谱在共振波长附近具有不对称且尖锐的谐振峰,传输光的强度在共振波长附近从0突变为1,该机制可显著提高硅基光开关、探测器、...     

点缺陷二维光子晶体波导的出射光集束

光子晶体器件在高密度集成光通信中有广泛的应用,为解决光子晶体波导出射光场的空间控制,采用时域有限差分法分析光子晶体波导结构的缺陷传播特性,提出基于点缺陷优化波导结构,通过在波导出射口两侧加上点缺陷,出射光方向性有显著提高,实现三点光源干涉系统的光集束。模拟结果表明缺陷态越靠近能带结构中央,共振腔的耦合效率越高;相反,缺陷态越靠近能带结构边缘位置,则共振腔耦合效率越低,因此,选取禁带区域四分之一处对应的点缺陷,可以有效实现波导出射的光集束。     

点缺陷二维光子晶体波导的出射光集束

光子晶体器件在高密度集成光通信中有广泛的应用,为解决光子晶体波导出射光场的空间控制,采用时域有限差分法分析光子晶体波导结构的缺陷传播特性,提出基于点缺陷优化波导结构,通过在波导出射口两侧加上点缺陷,出射光方向性有显著提高,实现三点光源干涉系统的光集束。模拟结果表明缺陷态越靠近能带结构中央,共振腔的耦合效率越高;相反,缺陷态越靠近能带结构边缘位置,则共振腔耦合效率越低,因此,选取禁带区域四分之一处对应的点缺陷,可以有效实现波导出射的光集束。     

Enlargement of the band gap in the metal-insulator-metal heterowaveguide

We present a new metal-insulator-metal (MIM) heterowaveguide to enlarge the band gap, which is formed by alternately stacking two kinds of metals, modulating the MIM waveguide slit, and inserting different dielectric materials with the effective refractive index periodically modulated. Based on this structure, we adopt two different methods to enlarge the band gap: changing the thickness of the unit layer and combining two MIM structures. Both of them widen the band gap when surface plasmon polaritons propagate through the structure. This metal heterostructure is expected to have applications in surface plasmon polaritons (SPPs) based optical devices, such as filters, waveguides, especially for broad band gap elements.     

Unidirectional manipulation of surface plasmon polariton by dual-nanocavity in a T-shaped waveguide

A structure of two dimensional T-shaped metal-insulator-metal waveguide with dual-nanocavity is proposed. The two nanocavities located at each side of the slit on the lower metallic surface, act as band rejection filters and are capable of stopping the surface plasmon polaritons (SPPs) at the resonant wavelengths. The Fabry-Perot interferometry theory and the Finite-Difference-Time-Domain method are utilized to investigate the proposed waveguide. The numerical results demonstrate the realization of miniaturized photonic devices for effectively switching the SPPs propagation between the left and right waveguides in one direction.